The International Consultative Committee on Telephony and Telegraphy (CCITT, now International Telecommunication Union—Telecommunication (ITU-T)) V.42bis standard, published as Recommendation V.42 bis in Geneva in 1990, is an addition to the V.42 error-correction protocol for modems. The purpose of the addition is to increase data throughput using a data compression procedure. As defined in the standard, the compressed operation has two modes: a “compressed mode” in which data is transmitted in code words, and a “transparent mode” in which data is transmitted in uncompressed form.
According to the standard, an encoder compatible with the V.42bis standard will switch between these modes on the basis of “data compressibility testing, in which the efficiency of the encoding process is estimated and transparent mode or compressed mode selected to maximize efficiency” (section 7.1f) of Recommendation V.42bis). The standard then states: “The data compression function shall periodically apply a test to determine the compressibility of the data. The nature of the test is not specified in this Recommendation; however it would consist of a comparison of the number of bits required to represent a segment of the data stream before and after compression.” (section 7.8 of Recommendation V.42bis).
An encoder compatible with V.42bis would therefore require an implementation of the data compressibility test.
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